1. Field of the Invention
The present invention relates to a fuel injection nozzle and more particularly to a variable injection hole type fuel injection nozzle.
2. Description of the Related Art
Extreme importance has been directed to NOx reduction in the low-speed, light-load region and to smoke reduction in the high-load region of a diesel engine. In order to cope with the former, it is preferred to reduce the initial injection rate by effecting fuel injection for a good length of time using small-diameter injection holes and to establish optimum burning condition by accelerating fuel atomization, whereas in order to solve the latter, it is preferred to effect fuel injection for a short time using large-diameter injection holes.
However, conventional fuel injection nozzles of the sort referred to above make it impossible to deal with the problems that have been posed so far.
In order to take steps to deal with the aforementioned problems, there has been proposed a variable injection nozzle designed for the injection hole area to be made variable and for the injection hole to be made switchable as desired by means of an actuator. An injection nozzle of such a type that has been proposed in Japanese Patent Unexamined Publication No. Sho 60-36772 is of such a translation type that injection holes are controlled by moving a valve in the axial direction.
In this prior art, a first and a second injection hole group are provided at different levels in the hole wall in the leading end portion of a nozzle body, whereas a through-hole is formed on the axial line of the hole of the nozzle body. Moreover, a spool valve formed with a land is inserted through the through-hole so as to make a rod portion face the position of the second injection hole group, and the spool valve is moved axially by an actuator (electromagnetic solenoid) provided above the nozzle body.
However, there arises a problem from the aforementioned construction in that because the lower end of the spool valve is made a surface where the internal pressure of the engine is received, control force great enough to hold the position of the spool valve against the shaft power due to the pressure in the engine cylinder is needed. Consequently, not only the spring and the actuator but also the injection nozzle tend to become large-sized.
Another problem inherent in the prior art is that since the edge face of the spool valve is exposed in the engine cylinder, seizing at high temperatures due to the burning of the fuel in the cylinder occurs or stable operation is easily ruined as free carbon sticks and accumulates.
While the diameters of the injection holes in the first and second prior art injection hole groups are made equal, fuel is jetted from the first injection hole group when the needle valve is opened and the second injection hole group is also opened via the rod portion at the time the actuator and the spool valve are lifted so as to jet the fuel from both the first and second injection hole groups. As a result, the first injection hole group is always kept open at the time of low and high loads, the problem in this case is that since the plurality of injection hole groups are uncontrollable individually it is impossible to make the diameters of the injection holes variable so as to effect optimum atomization.